38 Oilfield Review

Basin to Basin:Plate Tectonics in Exploration

The principles of plate tectonics help explorers understand and evaluate hydrocarbon

plays. Since the start of the 21st century, these ideas have been successfully applied

to presalt basins and turbidite fans along the coasts of South America and western

Africa. Guided by global plate tectonics, exploration companies are applying winning

play strategies from one coast of the South Atlantic to discover and prove similar

plays on the opposite coast.

Ian BryantNora HerbstHouston, Texas, USA

Paul DaillyKosmos EnergyDallas, Texas

John R. DribusNew Orleans, Louisiana, USA

Roberto FainsteinAl-Khobar, Saudi Arabia

Nick HarveyNeftexAbingdon, England

Angus McCossTullow Oil plcLondon, England

Bernard MontaronBeijing, People’s Republic of China

David QuirkMaersk OilCopenhagen, Denmark

Paul TapponnierNanyang Technological UniversitySingapore

Oilfield Review Autumn 2012: 24, no. 3. Copyright © 2012 Schlumberger.For help in preparation of this article, thanks to Steve Brown, Copenhagen, Denmark; George Cazenove and Jonathan Leather, Tullow Oil plc, London; James W. Farnsworth, Cobalt International Energy, Inc., Houston; Winston Hey, Houston; Susan Lundgren, Gatwick, England; and Richard Martin and Mike Simmons, Neftex, Abingdon, England.Petrel is a mark of Schlumberger.

New discoveries often emerge from previous suc-cesses. Once a play concept has proved commer-cially viable, oil companies are able to apply characteristics from their play to a regional or global framework in search of other accumula-tions. Through integration of exploration infor-mation, drilling data and geologic models from a successful play and through application of plate tectonic models, geoscientists are finding analog plays across ocean basins.

From the North Sea to the Gulf of Mexico and from offshore South America to offshore Africa, explorationists have discovered major oil and gas fields in continental margin systems. The Santos, Campos and Espirito Santo basins off the coast of Brazil contain prolific oil discoveries, and the application of plate tectonic concepts has enabled explorers to extend that play across the Atlantic to offshore western Africa. Within the last few years, exploration companies have applied prin-ciples of plate tectonics to extend and relate upper Cretaceous turbidite fan plays westward—from West Africa across the Equatorial Atlantic to French Guiana and Brazil. This article describes some of the fundamental concepts that today’s geoscientists use to extrapolate plays across ocean basins. Case studies demonstrate how explorers have used plate tectonics and regional geology to expand exploration efforts in both directions across the Atlantic Ocean.

Basic ConceptsBasins, petroleum systems and hydrocarbon plays are vital concepts in petroleum exploration. Basins collect the sediments that become the building blocks for petroleum systems. A petroleum system comprises an active source rock and the oil and gas derived from it that migrate to a reservoir and become confined there by a trap and seal.1 A play is a model used to explore for hydrocarbon depos-its having similar characteristics. Petroleum sys-tems may contain one or more plays, depending on the reservoir and style of trapping mechanism.2

Exploration experts systematically apply these concepts to locate prospects for drilling. Software platforms for databases, data integration and modeling are helping experts optimize their explo-ration workflows.

A basin is a depression in the Earth’s surface that accumulates sediments. Basins form when the Earth’s lithosphere is stretched, fractured, loaded down or compressed in response to global tectonic processes. These processes also govern the size and depth—the accommodation space—of a basin, while climatic conditions determine water and sediment input for the basin fill material.

Autumn 2012 3939

Basins may be deformed by tectonic motion: extension, compression, strike-slip motion or any combination thereof. Extension may cause normal faulting and may be accompanied by stretching, thinning and sagging of the crust. Compression results in thrust faulting, folding, shortening and thickening. Strike-slip motion gives rise to translation and lateral faulting. A combination of these phenomena produces

1. Al-Hajeri MM, Al Saeed M, Derks J, Fuch T, Hantschel T, Kauerauf A, Neumaier M, Schenk O, Swientek O, Tessen N, Welte D, Wygrala B, Kornpihl D and Peters K: “Basin and Petroleum System Modeling,” Oilfield Review 21, no. 2 (Summer 2009): 14–29.Stewart L: “The Search for Oil and Gas,” Oilfield Review 23, no. 2 (Summer 2011): 59–60.

2. Doust H: “Placing Petroleum Systems and Plays in Their Basin History Context: A Means to Assist in the Identification [of] New Opportunities,” First Break 21, no. 9 (September 2003): 73–83.Doust H: “The Exploration Play: What Do We Mean By It?,” AAPG Bulletin 94, no. 11 (November 2010): 1657–1672.

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pull-apart basins, push-up blocks and transten-sion or transpression oblique slip. Thus, localor large-scale movements provide the impetusfor creation of stratigraphic or structuraltraps. Stratigraphic traps result from facieschanges or juxtaposition of impermeable andpermeable strata. Structural traps form as aresult of strata deformation. The tectonic andstratigraphic history of a basin gives it a globaland regional setting for its formation, fillingand deformation.3

Exploration teams composed of geologists, geo-chemists, paleontologists, geophysicists and petro-physicists unravel the history of a basin andsequence of tectonic events and cycles of sedimen-tation filling a basin. They identify source rockswithin the basin and correlate them with knowntrapped hydrocarbons.The teams examine the geo-logic elements and processes that created knownsource rocks and traps to develop leads to othersimilarly generated accumulations (above). Afterfurther investigation, if the lead still appears to

have potential to trap hydrocarbons, it becomesa prospect.4

Once identified, the prospects are rankedaccording to uncertainty, risk, potential rewardand market value of hydrocarbons.

Integrated software systems that incorporatemapping and petroleum systems and play analy-sis tools, such as the Petrel E&P platform, helpgeoscientists evaluate basins (next page).5

Geoscientists use them to construct and sharegeologic models in 3D and provide an environ-ment for storing data and models.

> Petroleum systems. Explorationists define the petroleum system as the geologic elements and processes that areessential for the existence of a petroleum accumulation. This cross section summarizes petroleum systems along a SouthAtlantic continental margin. The geologic elements must be present in the following order: The source rock containsorganic matter, reservoir rock receives the hydrocarbons and has sufficient porosity and permeability for storage andrecovery of hydrocarbons, sealing caprock is impermeable to keep the fluids in the reservoir and overburden rock buriesthe source rock to depths having the optimal temperature and pressure for source rock maturation and hydrocarbongeneration. Rifting of the South Atlantic Ocean started with extension and faulting (black solid going to dashed lines) ofcontinental crust (brown). The continental crust thinned and eventually split apart. As the two parts of the continental crustseparated (only the right side is shown here), oceanic crust (gray) formed at a midocean ridge (not shown) during seafloorspreading. The continental margin is located where the thinned continental crust meets oceanic crust. Synrift lacustrinebasins were preserved and filled with source (blue) and reservoir (white) rock that were eventually trapped and sealedunderneath salt (purple). Hydrocarbons from synrift source rock migrated to limestone reservoirs (green bricks) that wereburied and trapped beneath postsalt marls (green). The marls also provided source rock (dark green). During the Tertiary,clayey-sandy sediments (yellow and tan) buried the margin, providing source rock, reservoirs, caprock and overburden.[Illustration adapted from Huc AY: “Petroleum in the South Altantic,” Oil & Gas Science and Technology—Revue de l’InstitutFrançais du Pétrole 59, no. 3 (May–June 2004): 243–253.]

Clayey-sandysediments

Marls

Limestone

Salt

Synriftlacustrinesediments

Oceaniccrust Continental crust

Lithosphere

C

O

C

R

CR

O

Terti

ary

Cret

aceo

us

C

C

C

R

R

OverburdenCaprockReservoirsSource rocks

Autumn 2012 41

By creating models at various scales, geo-scientists are able to develop geocellular mod-els from global to regional and local scales.This integration allows geoscientists to deter-mine, for example, whether a particular localchannel-levee interpretation is consistent withthe regional interpretation or whether a wide-spread organic-rich facies mapped at the tec-tonic plate scale corresponds to source rockfacies in the prospect model of the targetedpetroleum system.

3. A facies is a rock unit defined by characteristics thatdistinguish it from neighboring units.For more on stratigraphic and structural traps:Caldwell J, Chowdhury A, van Bemmel P, Engelmark F,Sonneland L and Neidell NS: “Exploring for StratigraphicTraps,” Oilfield Review 9, no. 4 (Winter 1997): 48–61.For sequence stratigraphy: Neal J, Risch D and Vail P:“Sequence Stratigraphy—A Global Theory for LocalSuccess,” Oilfield Review 5, no. 1 (January 1993): 51–62.

> Exploration software platform. Exploration experts combine seismic information, well logs, geochemical and heat flow data and other geologic data towork from basin to prospect scale (clockwise top center to middle right). Regional to prospect scale models of traps (top right) and reservoirs (middle right)built in the Petrel platform benefit from integration with structural restoration tools (bottom right) and petroleum system modeling (bottom center). Bothpetroleum system modeling and structural restoration tools may be used to gain an understanding of the geomechanics of the basin to guide evaluation ofseals (bottom left) and plan exploration wells. Risk assessment tools allow exploration teams to assign uncertainty and risk to acreage and drillableprospects (middle left). Petroleum economic evaluation enables planning exploration portfolios (top left).

Project and portfolio economics Model-based interpretation from basin to prospect Trap

Play and prospect evaluation Reservoir

Geomechanics and seal analysis Charge and timing petroleum system modeling Structural restoration

4. This chain of events from hydrocarbon source to itsresting place in a distant reservoir applies toconventional petroleum systems. For unconventionalsystems, the source rock may also be the reservoir rock.Such unconventional systems include oil and gas fromshale or coalbed methane.McCarthy K, Rojas K, Niemann M, Palmowski D, Peters Kand Stankiewicz A: “Basic Petroleum Geochemistry forSource Rock Evaluation,” Oilfield Review 23, no. 2(Summer 2011): 32–43.

5. Al-Hajeri et al, reference 1.

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Because these various input data are con-strained by a stratigraphic model, the geocellularmodels are displayed not only in true verticaldepth (TVD) or two-way traveltime, but also ingeologic time (above). In addition, geologists areable to project characteristics of a given strati-

graphic interval to analogous strata in conjugatebasins or in frontier areas. Geologists are alsoable to use qualities from a data-rich region todevelop a sequence stratigraphic context for pre-dicting facies in data-poor regions.

Plate Boundaries and Rifted andTransform MarginsPlate tectonic science has established that theEarth’s outermost layer, the lithosphere, com-prises a number of major and many minor platesthat move relative to one another (next page).6

> South Atlantic conjugate margins through geologic time. Two regional geologic models, built on opposing coasts of the South Atlantic, are constrained bya global sequence stratigraphic model. By assimilating interpretations into a 3D environment using the Petrel platform, geoscientists have derived aworkflow to populate a tectonic plate–scale geocellular model for the sedimentary evolution of the margins through geologic time as illustrated in theexploded view of the South Atlantic continental margins from Precambrian time at the deepest surface to the present at the upper surface. Data assembledin this way on a common software platform allow explorationists to project petroleum system facies to a data-poor region by using sequence stratigraphyand elements of petroleum system modeling from a data-rich region to correlate and extrapolate associated facies. A recent example of this approach maybe found along the transform margin where successful exploration concepts developed in Turonian-age lowstand turbidite fans offshore Ghana have beenapplied offshore French Guiana, leading to the recent Zaedyus discovery within similar deposits. Visualized in geologic time, these lowstand systems maybe explored with their associated petroleum elements. Compelling evidence from wireline log responses, hinterland cooling events and biostratigraphicallyconstrained unconformities were integrated; the results suggest that Campanian-age lowstand deposits may also provide attractive reservoir targets in theGuyana-Suriname basin offshore northern South America. The Campanian stratigraphic interval, while not as well tested as the Turonian interval, has alsobeen attracting interest on the African margin offshore Ghana, Liberia and Côte d’Ivoire. (Illustration used with permission from Neftex.)

Jubilee discovery,Tano basin

Present day

Azul and Cameia discoveries,Kwanza basin

Cretaceous

Precambrian

CretaceousPresent day Precambrian

Zaedyus discovery,Guyana-Suriname basin

Play projection

Tupi discovery,Santos-Campos basin

Extrusive volcanicsNondepositionOrganic-rich clasticsLacustrine faciesDeep marine sand-dominated clasticsParalic faciesDeep marine carbonatesShallow marine carbonatesDeep marine clasticsShallow marine clasticsTerrestrial sediments

Autumn 2012 43

This motion is driven by the convection and flowof hot ductile material in the mantle underlyingthe lithosphere. The lithosphere consists of twolayers: the crust and the lithospheric mantle.7

The crust is further divided into two categories.Continental crust is mostly of granitic composi-tion; its density averages about 2.7 g/cm3, and itsthickness is about 35 km [22 mi] in most placesbut ranges from 20 to 70 km [12 to 43 mi].Oceanic crust has a basaltic composition and is

denser and thinner than continental crust. Itsdensity averages about 2.9 g/cm3, and its thick-ness ranges from 5 to 10 km [3 to 6 mi]. Thehigher density of the oceanic crust causes it torest lower in the mantle than continental crust.

Over geologic time, tectonic plate motionshave amalgamated small continents to form super-continents and separated them again into a collec-tion of smaller continents distributed across theplanet. The most recent giant supercontinent,

Pangea, formed during the Paleozoic era, then wasrifted apart beginning about 225 to 200 millionyears ago [Ma]. The breakup started with Pangeaseparating into the Laurasia and Gondwana super-continents in the north and south, respectively.The subsequent breakup of Laurasia andGondwana resulted in the opening of the Atlanticand Indian oceans and evolved to the present dayconfiguration of continents and oceans.

> Plates. The Earth’s lithosphere is divided into numerous plates. Relative motion of the plates (arrows) determines whether the plate boundaries areconvergent, transform or divergent. [Map adapted from “Interpretative Map of Plate Tectonics,” an inset to Simkin T, Tilling RI, Vogt PR, Kirby SH,Kimberly P and Stewart DB: “This Dynamic Planet—World Map of Volcanoes, Earthquakes, Impact Craters, and Plate Tectonics,” US Geological Survey,Geologic Investigations Series Map I–2800 (2006).]

Eurasia plate

Pacific plate

North America plate

Eurasia plate

Anatolia plate

Africa plate

Antarctica plateAntarctica plate

Scotia plate

Antarctica plate

Convergent boundary barbs pointto direction of convergence Major transform boundaryPossible boundary Divergent boundary Plate movement

Arabiaplate

Indiaplate

Australia plate

Australiaplate

Pacific plate

Nazca plate

Cocosplate

South America plate

Juan de Fucaplate

Philippineplate

Caribbean plate

6. The lithosphere is the 50 to 200 km [30 to 120 mi] thick,rigid outer layer of Earth; its thickness is determined bythe depth of the brittle-to-ductile transition temperature,which is roughly 1,000°C [1,800°F]. The upper part of thelithosphere is the crust and the lower part is thelithospheric mantle.

For more on plate boundaries: Bird P: “An UpdatedDigital Model of Plate Boundaries,” GeochemistryGeophysics Geosystems 4, no. 3 (March 2003),http://dx.doi.org/10.1029/2001GC000252 (accessedAugust 21, 2012).

7. Earth’s mantle is the 2,900 km [1,800 mi] thick layer thatlies between Earth’s crust and outer core. The mantle isdivided into the upper mantle, transition zone and lowermantle. The upper mantle is about 370 km [230 mi] thickand divided into the lithospheric mantle and theasthenosphere.

44 Oilfield Review

The plates move relative to one another andinteract with each other at their boundaries(left). The three types of plate boundaries are thefollowing: convergent, or compressional; trans-form, or strike slip; and divergent, or extensional.

At convergent plate boundaries, plates movetoward one another. Plates respond in a numberof ways when they collide, depending on whetherthe convergence is continent to continent, oceanto ocean or ocean to continent. Continent-to-continent convergence—collision—results inshortening and thickening of the crust. The colli-sion between the Indian and Asian continents isone example. This convergence created theHimalaya Mountains and Tibetan Plateau andresulted in the southeastward lateral escape ofSundaland and southeast China in the directionaway from the collision between India and Asia.8

Ocean-to-ocean or ocean-to-continent conver-gence results in subduction: one oceanic platedives under the other plate. An example of ocean-to-ocean convergence occurs at the MarianasTrench, where the Pacific plate plunges west-ward under the small Philippine plate in thewestern Pacific Ocean. Ocean-to-continent con-vergence occurs along the western AndesMountains, where the Pacific plate dives east-ward under the South America plate.

At transform boundaries, plates slide pasteach other, which occurs along the San AndreasFault in California, USA. This fault accommo-dates movement of the Pacific plate northwardpast the North America plate. The North andEast Anatolian faults in Turkey are also trans-form boundaries. These faults accommodate thewestward movement of the Anatolia platetoward the Mediterranean Sea as it escapes thecompression between the converging Eurasiaand Arabia plates.

At divergent plate boundaries, a plate splits,forming two smaller plates that move apart fromeach other. Divergent plate boundaries may startout as continental rift systems; in millions ofyears, these land-based rifts become oceanic rifts.Examples of modern-day continental rifts are theEast African rift; the Lake Baikal rift, Russia; andthe Basin and Range Province, western USA.

In continental rifts, the crust undergoesextension, faulting and thinning until it splits. Atthe split, a volcanic ridge forms as hot mantlematerial wells up to fill the void left by the sepa-rating plates. The mantle material of basalticcomposition accretes to the plate edges, coolsand forms new oceanic crust. As the plates moveapart, the oceanic crust grows, building an oceanthat widens between the slowly separating plates.The process is called seafloor spreading. The Red

>Midocean ridge and transform fault plate boundary. Midocean spreading (white and red arrows)rarely occurs along a single clean rift zone. Here, the divergent plate boundary (dashed yellow line)consists of two segments of a midocean ridge connected by a transform fault. In the transform fault,or the active part of the fracture zone between the ridge segments, the plates slide past each otherin opposite directions (black opposing arrows). In the inactive part of the fracture zone, outside of theridge segments, the plate sections are locked together and move in the same direction (black parallelarrows). (Adapted from Garrison TS: Oceanography: An Invitation to Marine Science, 4th ed. PacificGrove, California, USA: Brooks/Cole Publishing Company, 2002.)

Ocean crust

Midoceanridge

Plate boundary

Plate boundary

Fracturezone(inactive)

Transform fault(active part offracture zone)

Fracturezone(inactive)

Lithosphere

Oceanic crust

Asthenosphere

> Plate boundaries. Earth’s lithospheric plates move relative to one another. This movement isaccommodated along plate boundaries. Convergent boundaries occur where plates move toward oneanother. One plate may subduct—dive—under another; trenches mark the line of the bending,subducting plate. Chains of island arc stratovolcanoes may form along subduction zones above thedowngoing plate. Transform boundaries occur where plates slide past one another; oceanic transformfault zones transfer seafloor spreading from one midocean ridge segment to another. Divergent plateboundaries occur where plates split apart at seafloor spreading ridges and continental rift zones. Hotspots occur where plumes of hot mantle material impinge on lithospheric plates; they may induceshield volcanoes and cause flood basalts to pour out over plates (not shown). [Image adapted from“Schematic Cross Section of Plate Tectonics,” an inset to Simkin T, Tilling RI, Vogt PR, Kirby SH,Kimberly P and Stewart DB: “This Dynamic Planet—World Map of Volcanoes, Earthquakes, ImpactCraters, and Plate Tectonics,” US Geological Survey, Geologic Investigations Series Map I–2800 (2006).]

Convergentplate boundary

Trench Shieldvolcano

Hot spot

AsthenosphereOceanic crust

Lower mantle

Upper mantleUpper m

antleContinental crust

Subductingplate

Lithosphere

Island arcstratovolcano

Trench

Transformplate boundary

Divergentplate boundary

Oceanic spreadingridge

Convergentplate boundary

Continental rift zone(young plate boundary)

Transform boundaryConvergent boundary

Plate

Asthenosphere

Divergent boundary

Autumn 2012 45

Sea and Gulf of Aden rift that separates theAfrica and Arabia plates is a young divergentplate boundary. The Mid-Atlantic Ridge, whichencompasses the midocean rift and ridge thatseparates the Americas from Europe and Africa,is a mature divergent plate boundary.

As continents move apart, they rarely do soalong a single separation zone or rift. Rather,the rift is a series of segments offset by trans-form faults and fracture zones. Transform faultsare strike-slip faults that connect rift segments.They transfer the spreading motion or accom-modate spreading rate differences between riftsegments; they are active only between rift seg-ments.9 Transform faults leave scars on theocean floor called fracture zones. Transformfaults and fracture zones are oriented perpen-dicular to the midocean ridge and parallel tothe spreading direction; they mark the path ofplate movement as the rifted continental mar-gins move farther apart.

The ages and thermal histories of oceanicrocks differ on opposite sides of transform faults.Along the fault, younger, hotter and lower densityrocks are juxtaposed against older, colder andhigher density rocks. Because they are hotter, theyounger rocks are thermally uplifted to a higherelevation than their older, cooler and densercross-fault neighbors, causing a difference inocean floor elevation on either side of the fault.These elevation differences may remain as therocks cool, leaving scars—fracture zones.Because the fracture zones are nearly parallel tothe midocean ridge spreading direction—thedirection of relative plate motion—they leavetracks of the opening of the ocean (previouspage, bottom).

As seafloor spreading continues, previouslyconnected continental margins move fartherapart. A continental margin, where continentalcrust meets or transitions to oceanic crust, is arelic of faulting during continental breakup.Thus, continental margins that face a midoceanrift commonly have overlaps and may also havetransform and rifted margin segments. Transformmargins occur where continents break up andseparate by shear movement along transformstrike-slip faults. Rifted margins form where con-tinents break up and separate by extensionalmovement perpendicular to coastlines and alongdip-slip faults.

Gondwana BreakupThe relative movement of adjacent tectonicplates throughout geologic time has been quanti-fied by remote-sensing technologies. For conti-nents, scientists determine plate movement by

fitting apparent polar wander curves.10 Foroceans, scientists determine plate movementfrom magnetic anomaly patterns caused bynorth-to-south polarity reversals of Earth’s mag-

netic field and from fracture zones on the oceanfloor (below).11 However, there are no useful mag-netic anomalies to constrain the Gondwanabreakup history during the Cretaceous period

8. Sundaland refers to the Sunda shelf region of SoutheastAsia, which includes Malaysia, Sumatra, Java andBorneo. For more about the lateral escape of SoutheastAsia and Sundaland: Tapponnier P, Lacassin R, LeloupPH, Schärer U, Zhong D, Wu H, Liu X, Ji S, Zhang L andZhong J: “The Ailao Shan/Red River Metamorphic Belt:Tertiary Left-Lateral Shear Between Indochina and SouthChina,” Nature 343, no. 6257 (February 1, 1990): 431–437.

9. Strike-slip displacement or motion refers to the horizontalmovement of the other side of the fault relative to thereference side—the side on which one is standing,facing the fault. The motion is right lateral when theother side of the fault moves to the right and left lateralwhen the other side moves to the left.

10. For more on plate motions and polar wander: Besse Jand Courtillot V: “Apparent and True Polar Wander andthe Geometry of Geomagnetic Field Over the Last 200Myr,” Journal of Geophysical Research 107, no. B11(November 2002): EMP 6-1 to 6-31.

>Magnetic anomalies and seafloor spreading. Scientists obtained evidenceof seafloor spreading by determining the polarity of magnetic anomalies onboth sides of midocean ridges. Earth’s magnetic field changes its polarityfrom time to time. The ocean floor is youngest and hottest at the oceanicridge spreading center and becomes progressively older and cooler towardthe continent-ocean boundary. As the ocean floor rocks and theirferromagnetic minerals cool below the Curie temperature, the ferromagneticminerals become magnetized in the direction consistent with the existingpolarity of Earth’s magnetic field. Rocks displaying dominantly normalpolarity, equivalent to present-day magnetism, are shown by black stripes onthe plate cross section. Rocks with dominantly reverse polarity magnetismare shown as white stripes. The symmetry of the magnetic anomaly stripingon either side of the ridge demonstrates the movement of the seaflooraway from the spreading center. Dating each polarity shift—normal toreverse and reverse to normal—turns the magnetic anomaly map into anmagnetochronology map for seafloor spreading; the age of each reversal isan isochron (white lines)—a contour of time—and the time interval betweenmagnetic reversals is a magnetic chron (MC), during which Earth’s magneticfield is dominantly, or constantly, one polarity.

Seafloor spreading

Magnetic chrons

Oceaniccrust

Plate temperature and age

Hot andyoung

Coldand old

Lithosphere

MC6 MC5 MC5 MC6MC4 MC4MC3 MC3MC2 MC2MC1 MC1

Reverse polarity

Normal polarity

Isochrons

Midocean ridge

Isochrons

Besse J and Courtillot V: “Correction to ‘Apparent andTrue Polar Wander and the Geometry of GeomagneticField Over the Last 200 Myr,‘” Journal of GeophysicalResearch 108, no. B10 (October 2003): EMP 3-1 to 3-2.

11. For more on plate motions, magnetic anomalies andseafloor spreading: Hellinger SJ: “The Uncertainties ofFinite Rotations in Plate Tectonics,” Journal ofGeophysical Research 86, no. B10 (October 1981):9312–9318.Karner GD and Gambôa LAP: “Timing and Origin of theSouth Atlantic Pre-Salt Sag Basins and Their CappingEvaporates,” in Schreiber BC, Lugli S and Babel M (eds):Evaporites Through Space and Time. London:The Geological Society, Special Publication 285(January 2007): 15–35.

46 Oilfield Review

from roughly 120 to 84 Ma because Earth’s mag-netic field was stable and did not experiencemagnetic polarity reversals during that interval.12

Nonetheless, through dating of the flood basaltsthat poured over the Gondwana continent, geo-scientists generally agree that the breakup of theGondwana supercontinent, which resulted in theopening of the South Atlantic Ocean and theseparation of the South America and Africaplates, started about 130 Ma during the EarlyCretaceous epoch. The breakup started in thesouth, moved progressively north and was com-pleted about 20 to 30 million years later duringthe Aptian to Albian geologic ages.13 The central

segment opened later because the continentalplate was hotter and softer there. Consequently,it stretched further and reached a higher eleva-tion because of thermal uplift before breakup.

The South Atlantic Ocean extends from theMarathon Fracture Zone (FZ) in the north to theAntarctic Plate in the south and may be dividedinto four segments separated by major FZs thatcross the Atlantic Ocean (above).

Adjacent to the Rio Grande FZ, the RioGrande Rise and the Walvis Ridge originated fromthe Tristan da Cunha hot spot that is responsiblefor the Paraná and Etendeka flood basalts inBrazil and Namibia, respectively.14 When theocean opened, the Rio Grande Rise and Walvis

Ridge formed as the South America plate driftedto the NW and the African plate drifted NE rela-tive to the Tristan da Cunha hot spot. The result-ing ridges formed a broad volcanic high thatisolated the central segment of the South AtlanticOcean from encroachment by marine water fromthe southern segment.

The basin filling histories of the central andsouthern segments of the South Atlantic differfrom one another.15 In particular, the central seg-ment is dominated by thick salt basins that formedduring the Aptian age (125 to 112 Ma), whereasthe continental margins of the southern segmentsubsided at the margins of an open ocean.

> Tectonic map of the South Atlantic Ocean at the end of magnetic polarity chron 34 (MC34, 84 Ma). The red linerepresents the midocean ridge at the end of MC34. From north to south, the South Atlantic Ocean is divided into theEquatorial, Central, Southern and Falkland segments, bounded by the Marathon, Ascension, Rio Grande andAgulhas-Falkland fracture zones (FZs). The black dots show the approximate locations of the discoveries of Tupioffshore Brazil, Azul and Cameia offshore Angola, Jubilee offshore Ghana and Zaedyus offshore French Guiana.(Adapted from Moulin et al, reference 12.)

AFRICA

SOUTHAMERICA

Marathon FZ

Chain FZ

Romanche FZ

Potiguar basin

Ascension FZ

Kwanzabasin

Gabonbasin

Congobasin

Namibebasin

Namibiabasin

Rawsonbasin

Pelotasbasin

ParanáProvince

Sergipe-Alagoas

basin

WalvisRidge

EspíritoSantobasin

Camposbasin

Gulf ofGuinea

Santosbasin

Rio Grande FZ

Agulhas-Falkland FZ

Rio GrandeRise

Tristan da Cunhahot spot

GuineanPlateau

DemeraraPlateau

Equatorial Segment

Central Segment

Southern Segment

Falkland Segment

Cratons

Cretaceousvolcanism

MidoceanridgeAptian salt

Autumn 2012 47

The equatorial South Atlantic segment began to open later in the Early Cretaceous epoch—around 112 Ma.16 In its northern latitudes, this segment encompasses the Demerara plateau of Suriname and French Guiana and the Guinea pla-teau in West Africa. In its southern latitudes, it includes coasts of northern Brazil, Côte d’Ivoire and Ghana.17 The opening of the equatorial seg-ment, unlike the other segments, was not perpen-dicular to the continental margins because some of the plate motion was taken up by oblique movement or sideways tearing along faults.18

Geologists’ understanding of the geologic events that controlled geography, climate and basin history are based on the principles of plate tectonics. These principles form the foundation for developing exploration plays. Discoveries in the presalt and transform margin basins along the South American and western African coasts since 2006 illustrate these points.

Matching Salt Basins: From Brazil to AngolaThe Lula oil field—renamed from Tupi in 2010 to honor former Brazilian president Luiz Inacio Lula da Silva—was discovered in 2006 within

the Santos basin by Petróleo Brasileiro SA, or Petrobras.19 The discovery was made beneath Aptian salt on the Brazilian rifted margin of the central South Atlantic and established the pre-salt play.20

The presalt fields offshore Brazil are charged with hydrocarbons migrating from organic-rich source rocks deposited within anoxic lakes that developed around the time the South Atlantic was forming. At the start of the Aptian age, continental rifting ended and seafloor spreading began; how-ever, lake, rather than marine, conditions pre-vailed as the region was uplifted above the mantle plume of the Tristan da Cunha hot spot. In these lakes above the rifted continental margins, unusual carbonates were deposited during the Early Aptian (123 to 117 Ma). Similar to the pro-cess in present-day Lake Tanganyika in East Africa, shallow lacustrine carbonates were depos-ited during slow deepening of the lakes. Within the Early Aptian carbonates, the fossil record shows coquina strata overlain by microbialite strata as conditions changed from fresh to hypersaline water when the climate became more arid.21 These

carbonates form the reservoirs of Brazil’s Santos and Campos presalt basins.

With increased aridity during the Late Aptian (117 to 113 Ma), the basins became conducive to deposition of thick, 800- to 2,500-m [2,600- to 8,200-ft] layered evaporite sequences. Evaporites in the Santos basin show a history of rapid pre-cipitation of mostly halite from marine waters, followed by slow precipitation of complex salts. These later salts precipitated from highly con-centrated brines augmented by hydrothermal processes involving a fluid-rock chemical exchange with basaltic rock. The first 600 m[2,000 ft] of these evaporites are formed by two massive halite layers separated by a thin anhy-drite layer. The top of the evaporite sequence shows a number of deposition cycles with potas-sium- and magnesium-rich layered evaporites.22

This entire evaporite sequence precipitated in a deep rift lake behind the barrier created by the Walvis Ridge and Rio Grande Rise. This barrier was penetrated by deep fissures along which marine waters traveled, interacting chemically with the basaltic wall rock and leaking into the evaporating lake.

12. Torsvik TH, Rousse S, Labails C and Smethurst MA: “A New Scheme for the Opening of the South Atlantic Ocean and the Dissection of an Aptian Salt Basin,” Geophysical Journal International 177, no. 3 (June 2009): 1315–1333.Moulin M, Aslanian D and Unternehr P: “A New Starting Point for the South and Equatorial Atlantic Ocean,” Earth-Science Reviews 98, no. 1–2 (January 2010): 1–37.Blaich OA, Faleide JI and Tsikalas F: “Crustal Breakup and Continent Ocean Transition at South Atlantic Conjugate Margins,” Journal of Geophysical Research 116, B01402 (January 2011): 1–38.Cartwright J, Swart R and Corner B: “Conjugate Margins of the South Atlantic: Namibia–Pelotas,” in Roberts DG and Bally AW (eds): Regional Geology and Tectonics: Phanerozoic Passive Margins, Cratonic Basins and Global Tectonic Maps, Vol. 1c. Amsterdam, The Netherlands: Elsevier BV (2012): 202–221.Mohriak WU and Fainstein R: “Phanerozoic Regional Geology of the Eastern Brazilian Margin,” in Roberts DG and Bally AW (eds): Regional Geology and Tectonics: Phanerozoic Passive Margins, Cratonic Basins and Global Tectonic Maps, Vol. 1c. Amsterdam, The Netherlands: Elsevier BV (2012): 222–283.

13. Szatmari P: “Habitat of Petroleum Along the South Atlantic Margins,” in Mello MR and Katz BJ (eds): Petroleum Systems of South Atlantic Margins. Tulsa: The American Association of Petroleum Geologists, AAPG Memoir 73 (2000): 69–75.

14. Hot spots are surface manifestations of mantle plumes, which are stationary thermal anomalies that produce thin upwelling conduits of magma within the mantle. Hot spot volcanism yields flood basalts and long linear chains of volcanoes within tectonic plate interiors; along each chain, the volcanoes become progressively older in the direction of plate movement.Wilson M: “Magmatism and Continental Rifting During the Opening of the South Atlantic Ocean: A Consequence of Lower Cretaceous Super-Plume Activity?,” in Storey BC, Alabaster T and Pankhurst RJ (eds): Magmatism and the Causes of Continental Break-Up. London: The Geological Society, Special Publication 68 (1992): 241–255.

Quirk DG, Hertle M, Jeppesen JW, Raven M, Mohriak W, Kann DJ, Nørgaard M, Mendes MP, Hsu D, Howe MJ and Coffey B: “Rifting, Subsidence and Continental Break-Up Above a Mantle Plume in the Central South Atlantic,” in Mohriak WU, Danforth A, Post PJ, Brown DE, Tari GC, Nemcok M and Sinha ST (eds): Conjugate Divergent Margins. London: The Geological Society, Special Publication 369 (in press).

15. Séranne M and Anka Z: “South Atlantic Continental Margins of Africa: A Comparison of the Tectonic vs. Climate Interplay on the Evolution of Equatorial West Africa and SW Africa Margins,” Journal of African Earth Sciences 43, no. 1–3 (October 2005): 283–300.

16. Moulin et al, reference 12.17. The Guyanas, or Guianas, is the region of northern South

America that includes the nations of Suriname, Guyana and French Guiana. West Africa, or western Africa, is the westernmost region of the African continent and its southern edge extends along the northern coastline of the Gulf of Guinea and includes, from east to west, Nigeria, Togo, Benin, Ghana, Côte d’Ivoire, Liberia, Sierra Leone and Guinea.

18. Darros de Matos RM: “Tectonic Evolution of the Equatorial South Atlantic,” in Mohriak W and Talwani M (eds): Atlantic Rifts and Continental Margins.Washington, DC: American Geophysical Union, Geophysical Monograph 115 (2000): 331–354.Mascle J, Lohman P, Clift P and ODP 159 Scientific Party: “Development of a Passive Transform Margin: Côte d’Ivoire–Ghana Transform Margin—ODP Leg 159 Preliminary Results,” Geo-Marine Letters 17, no. 1 (February 1997): 4–11.Darros de Matos RM: “Petroleum Systems Related to the Equatorial Transform Margin: Brazilian and West African Conjugate Basins,” in Post P, Rosen N, Olson D, Palmes SL, Lyons KT and Newton GB (eds): PetroleumSystems of Divergent Continental Margin Basins. Tulsa: Gulf Coast Section, Society for Sedimentary Geology (2005): 807–831.

19. Beasley CJ, Fiduk JC, Bize E, Boyd A, Frydman M, Zerilli A, Dribus JR, Moreira JLP and Pinto ACC: “Brazil’s Presalt Play,” Oilfield Review 22, no. 3 (Autumn 2010): 28–37.

20. Presalt refers to before the formation or deposition of salt deposits. Presalt reservoirs are beneath salt deposits that have not flowed away from their place of deposition—beneath the autochthonous, or in place, salt. This definition differentiates presalt strata from subsalt or postsalt strata. For more: Beasley et al, reference 19.

21. Coquina is a limestone formed principally from shell fragments and indicates a nearshore environment with vigorous wave action. Microbialites, which are carbonate structures thought to be formed by microbes, have a range of shapes and sizes. They form in environments that are not conducive to the growth of corals.

22. Hardie LA: “On the Significance of Evaporites,” AnnualReview of Earth and Planetary Sciences 19 (May 1991): 131–168.Jackson MPA, Cramez C and Fonck J-M: “Role of Subaerial Volcanic Rocks and Mantle Plumes in Creation of South Atlantic Margins: Implications for Salt Tectonics and Source Rocks,” Marine and Petroleum Geology 17, no. 4 (April 2000): 477–498.Nunn JA and Harris NB: “Subsurface Seepage of Seawater Across a Barrier: A Source of Water and Salt to Peripheral Salt Basins,” Geological Society of America Bulletin 119, no. 9–10 (September–October 2007): 1201–1217.Nunn JA and Harris NB: “Erratum for ‘Subsurface Seepage of Seawater Across a Barrier: A Source of Water and Salt to Peripheral Salt Basins,’” GeologicalSociety of America Bulletin 120, no. 1–2 (January–February 2008): 256.

48 Oilfield Review

The necessary factors promoting such thicksalt accumulations were a rapidly sinking marginwith balance-filled basins or lakes behind anelevated outer volcanic high. This volcanic highwas a leaky barrier that restricted inflow of sea-water in an environment characterized by awarm, arid, desert climate (next page, bottom).23

Conditions were somewhat similar to present-dayconditions in the Dead Sea basin and in theDanakil Depression on the Afar Peninsula, north-east Africa.24 These layered salts form the seal forthe presalt reservoirs (See “Salt Deposition inActively Spreading Basins,” page 50).

The end of the Aptian age saw the finalbreaching of the Walvis Ridge–Rio Grande Risebarrier accompanied by flooding of marinewaters from the southern segment of the SouthAtlantic Ocean. These open marine conditionsallowed ocean waters to fill the basins of the cen-tral segment, halting any further evaporite depo-sition. Marine sediments formed on top of thesalt, starting with marine carbonates in theAlbian age (113 to 110 Ma). The postsalt sedi-mentation was controlled by continual openingand deepening of the South Atlantic by globalchanges of sea level. As the ocean opened, the

rifted margins tilted seaward, causing halokine-sis, in which the salt flows and deforms, givingrise to the salt structures that affected postsaltsediments where large volumes of oil were foundin the Campos basin (above).25

The Tupi discovery in 2006 opened up a newpetroleum play in the central South Atlantic, thepresalt play. Lula field lies in 2,126 m [6,975 ft] ofwater in the Santos basin Block BM-S-11 about250 km [155 mi] southeast of Rio de Janeiro. The1-RJS-628A discovery well was drilled to 4,895 m[16,060 ft] TVD subsea.26 The well flowed 780 m3/d[4,900 bbl/d] of oil and 187,000 m3/d [6.6 MMcf/d]

23. Davison I: “Geology and Tectonics of the South AtlanticBrazilian Salt Basins,” in Ries AC, Butler RWH andGraham RH (eds): Deformation of the Continental Crust:The Legacy of Mike Coward. London: The GeologicalSociety, Special Publication 272 (January 2007): 345–359.Lakes or basins are balance filled when the rate ofwater and sediment input is similar to the rate that theaccommodation space—area and depth—forms. Formore: Carroll AR and Bohacs KM: “StratigraphicClassification of Ancient Lakes: Balancing Tectonic andClimatic Controls,” Geology 27, no. 2 (February 1999):99–102.

24. Montaron B and Tapponnier P: “A Quantitative Model forSalt Deposition in Actively Spreading Basins,” Searchand Discovery Article 30117, adapted from an oralpresentation at the AAPG International Conference and

>Seismic lines across conjugate presalt rifted margins. These paired seismic lines are dip lines from the Santos basinoffshore Brazil (above) and the Kwanza basin offshore Angola (next page, top). The Santos basin seismic section is from ageneric 2D seismic line crossing close to the Lula field, a presalt discovery. The seismic section shows a nearly 2-km [1.2-mi]thickness of presalt sediments underneath the salt. The Kwanza basin section, offshore Angola, is from a 3D seismic surveyand shows a well-developed presalt section separated from postsalt sediments by complex salt geometries. (The Santosbasin section is used with permission from WesternGeco and TGS. The Kwanza basin section is used with permission fromWesternGeco and Sonangol.)

Postsalt sediments

Presalt

Basement

Salt

2 km

20 km

Exhibition, Rio de Janeiro, November 15–18, 2009.Bosworth W, Huchon P and McClay K: “The Red Seaand Gulf of Aden Basins,” Journal of African EarthSciences 43, no. 1–3 (October 2005): 334–378.Mohriak WU and Leroy S: “Architecture of RiftedContinental Margins and Break-Up Evolution: Insightsfrom the South Atlantic, North Atlantic and Red Sea–Gulf of Aden Conjugate Margins,” in Mohriak WU,Danforth A, Post PJ, Brown DE, Tari GC, Nemcok M andSinha ST (eds): Conjugate Divergent Margins. London:The Geological Society, Special Publication 369, http://dx.doi.org/10.1144/SP369.17 (accessed September 17,2012).

25. Halokinesis is the deformation of salt. Halokineticprocesses include downslope movement under gravityflow, expulsion and diapirism caused by overburdenloading and faulting resulting from tectonic stretching or

shortening. Salt deformation may cause deformation inthe strata deposited above it.Hudec MR and Jackson MPA: “Terra Infirma:Understanding Salt Tectonics,” Earth-ScienceReviews 82, no. 1–2 (May 2007): 1–28.Quirk DG, Schødt N, Lassen B, Ings SJ, Hsu D, Hirsch KKand Von Nicolai C: “Salt Tectonics on Passive Margins:Examples from Santos, Campos and Kwanza Basins,”in Alsop GI, Archer SG, Hartley AJ, Grant NT andHodgkinson R (eds): Salt Tectonics, Sediments andProspectivity. London: The Geological Society, SpecialPublication 363 (January 2012): 207–244.Beasley et al, reference 19.

26. Parshall J: “Presalt Propels Brazil into Oil’s FrontRanks,” Journal of Petroleum Technology 62, no. 4(April 2010): 40–44.

(continued on page 52)

W E

Autumn 2012 49

> Conditions conducive for thick salt accumulations. By the Aptian, about 120 Ma, the South Atlantic Ocean (map, center ) had scissored open from thesouth. The central segment of the South Atlantic was isolated from the open marine conditions of the southern segment by the Walvis Ridge (purple).The region was in an arid belt (between dashed white lines) where climate conditions were similar to those in the present-day Atacama desert, northernChile (bottom left ), and Kalahari desert, southern Africa (bottom right ). The central segment contained balance-filled basins and lakes. Under these climaticand isolated basin conditions, the basins and lakes became centers for precipitation of thick, layered salt sequences from basinal and hydrothermal brines,which were fed by marine water flowing through fractures in the leaky basaltic dam formed by the Walvis Ridge. (Map courtesy of CR Scotese, usedwith permission.)

Tropic of Capricorn

Salt basins

Arid belt

450 km

WalvisRidge

Present-day Kalahari DesertPresent-day Atacama Desert

Postsalt sediments

Presalt

Basement

Salt

2 km

20 km

WE

50 Oilfield Review

Salt Deposition in Actively Spreading Basins

Rifting, Spreading and TectonicsThe salt basins that face one another betweenthe Rio Grande Rise and the Gulf of Guineaare among the largest found alongPhanerozoic passive ocean margins (below).They formed during the Aptian (125 to110 Ma), during the opening stages of the cen-tral South Atlantic. The geometric, kinematicand temporal environment of this lowerCretaceous salt deposition appears strikinglysimilar to that of the Mid-Late Miocene RedSea (15 to 5 Ma).1

After the Tristan da Cunha hot spot inducedgiant volcanic eruptions that covered hugeareas of the African–South American litho-sphere with thick flood basalts about 143 Ma,the plates started to separate slowly at severalmillimeters per year. Narrow rifts, 50 to 80 km[31 to 50 mi] wide, which overlapped, formed

along the newborn plate boundary. Basaltic vol-canism and anoxic deepwater lakes—somedeeper than 1,000 m [3,300 ft], similar to LakeTanganyika today—punctuated the geology ofsuch rifts in the Late Hauterivian to EarlyBarremian (133 to 128 Ma).2

Continental separation was completed128 to 125 Ma. As full seafloor spreadingbegan, the rate of plate separation increasedto a few centimeters per year. The marinebasin, now 1,700 km [1,060 mi] long, 300 to500 km [190 to 310 mi] wide and 2 km[1.2 mi] deep, remained isolated between twolarge “dams” formed by the nascent equatorialAtlantic transform margin to the north andthe Walvis Ridge and Rio Grande Rise to thesouth. These dams restricted seawater flowinto the basin—flow that took place mostlyalong tectonic fissures through the southern

Walvis Ridge. Rapid evaporation of seawatercreated thick, layered evaporite deposits.Continuous open marine conditions were rees-tablished in the Early Albian (112 to 110 Ma).

Evaporites in the Santos BasinThree conditions are required to create athick, layered salt deposit: a basin about1,500 m [4,900 ft] deep, a continuous supply ofmineral-laden seawater and a warm and aridclimate. As evaporation takes place, the basinwater level drops quickly and stabilizes to acritical level: The evaporation rate equals thewater intake rate. The water salinity increasesgradually until the saturation concentration isreached for the least soluble salt mineral con-tained in the water.

Layers of calcite, dolomite and gypsum pre-cipitate—in that order—followed by halite(rock salt). Halite precipitates in quantitiesjust sufficient to maintain the water salinity atthe halite saturation level; this process can lastseveral thousand years to accumulate hundredsof meters of halite. If the climate becomes wet-ter, increased freshwater intake from rivers andrain may reduce the salinity enough to stophalite precipitation. For example, salinity maydrop back to the gypsum precipitation pointand eventually increase back to the halite pre-cipitation point. This is the layered sequenceobserved in the bottom 600 m [2,000 ft] ofSantos basin evaporites.

Water salinity levels may increase further,until they reach the saturation point at whichcomplex salts begin to precipitate. These saltsare potassium-, calcium- and magnesium-richevaporites such as sylvite, carnallite andtachyhydrite. Precipitation of complex saltsrequires an extremely arid climate and pre-cipitation may take a long time because thesehighly saline brines evaporate very slowly.During this process, the lake surface levelwill not change despite salt accumulatingon the lake bottom. The final result is a saltflat (next page).

> South Atlantic restoration. The Aptian, about 120 Ma, salt basin (purple) was 1,700 km [1,060 mi]long and restricted from open ocean conditions by the Tristan da Cunha hot spot (red circle) to itssouth and the embryonic equatorial Atlantic transform margin (opposing red arrows) to its north.The black arrows indicate the direction of plate movement. (Map courtesy of CR Scotese, usedwith permission.)

Aptian salt basin

Transform margin

Hot spot

SOUTH AMERICA

AFRICA

Autumn 2012 51

1. Mohriak WU and Leroy S: “Architecture of RiftedContinental Margins and Break-Up Evolution: Insightsfrom the South Atlantic, North Atlantic and Red Sea–Gulf of Aden Conjugate Margins,” in Mohriak WU,Danforth A, Post PJ, Brown DE, Tari GC, Nemcok Mand Sinha ST (eds): Conjugate Divergent Margins.London: The Geological Society, Special Publication369, http://dx.doi.org/10.1144/SP369.17 (accessedSeptember 17, 2012).Bosworth W, Huchon P and McClay K: “The Red Seaand Gulf of Aden Basins,” Journal of African EarthSciences 43, no. 1–3 (October 2005): 334–378.

2. Karner GD and Gambôa LAP: “Timing and Origin of theSouth Atlantic Pre-Salt Sag Basins and Their CappingEvaporates,” in Schreiber BC, Lugli S and Babel M(eds): Evaporites Through Space and Time. London:The Geological Society, Special Publication 285(January 2007): 15–35.

During the Aptian, South Atlantic saltbasins were located at latitudes correspond-ing to the arid belt that contains most of thesouthern hemisphere’s modern deserts. Theinitial evaporation rate was probably 2 m[7 ft] per year greater than the rainfall input,a rate currently observed in the Red Sea.4 Atan average halite deposition rate of 2 to 3 cm[0.8 to 1.2 in.] per year, it may have taken20,000 to 30,000 years to deposit the lower-most 600 m of Santos basin evaporites.5 Abovethat level, there are at least nine cycles con-taining complex salts, and these could havetaken 10 times longer to precipitate.Replacing water by salt doubles the weightapplied to the basin floor and accelerates sub-sidence. Approximately 30% of accommoda-tion space is gained in about 50,000 years byadding 500 m [1,600 ft] to the initial 1,500-m[4,900-ft] basin depth.

Observations from modern analogs such asLake Assal in the Afar region, Ethiopia, sug-gest seawater entered the salt basin throughfissures across the basaltic Walvis Ridge. Thisfissural process is also based on otherconsiderations:• The volumetric flow rate through cracks

must be small, as required by the salt pre-cipitation model.

• Because fissures in basalts can be up to ahundred meters deep, seawater flowingthrough fissures is less sensitive to varia-tions in ocean water level compared to thatrequired by flow over a dam.

• When the evaporation rate increases andthe basin level drops below the ocean level,the hydraulic-head difference will tend topromote flow through the fissures to main-tain the basin’s water level.

• The fractures provide a large contact sur-face between seawater and basalts, whichfavors the rock-to-fluid chemical exchangerequired for a chemical composition that iscompatible with complex salt deposition.6

Field observations and model results dem-onstrate that the deposition of thick, layeredevaporitic sequences requires a deep basin ina hot and arid climate with a continuous sup-ply of mineral-laden saltwater. These condi-tions must remain stable long enough forthick deposits to accumulate.

> Salt deposition sequence. During early rifting (1), freshwater lakes form on the stretchingcontinental margin. (The developing ocean is on the left side of each panel.) The ocean leveldrops and the lakes deepen (2) as the stretching continental margins thin and subside. The barrierthat separates the ocean from the lakes increases in relief with respect to the lake bottom. Sealevel rises (3), and seawater spills over the barrier and mixes with the lake water. About 123 Ma inthe Early Aptian (4), sea level falls by 50 m [80 ft] and isolates the basins from open ocean waters.The evaporation rate from the basins (5) is greater than the rate of water influx from rivers andrainfall and from seawater springs emanating from the leaky barrier; such leaks are the result offractures and fissures. The basin water level drops and water salinity gradually increases untilthe brine salinity level reaches the saturation concentration of the least soluble chemicalcomponent in the brine, which begins to deposit as a salt mineral (white, 6). During saltdeposition, salt layers (not shown) form as the brine chemistry changes. Salinity and saltsaturation concentrations depend on the climatic water balance within the basins and theseawater input to them through the leaky barrier. Salt mineral precipitation begins with the leastsoluble chemical component in the brine. This component precipitates until it depletes. Moresoluble components precipitate later. In this way, salt layers gradually build up and fill the basinsto form thick layered salt sequences. The final episode of salt deposition is marked by a terminalbrine (purple, 7) of high salinity, supersaturated with the least soluble component at the time.Finally, sea level rises sufficiently to inundate the continental margins (8); open marine conditionsare reestablished above the salt basins and such marine conditions shut down salt deposition.

1

2

3

4

5

6

7

8

Freshwater lakes form.

Freshwater lakes deepen. Ocean level falls.

Ocean level rises, spills over barrierand floods into freshwater lakes.

Ocean level falls.Fractured ridge allows hydraulic

communication between ocean and lake.

Basin level dropsas water evaporates.

Salt deposition starts.

Salt deposition ending.

Basin returns tofull marine conditions.

Terminal brine marks final salt deposition.

Montaron B and Tapponnier P: “A Quantitative Modelfor Salt Deposition in Actively Spreading Basins,”Search and Discovery Article 30117, adapted from anoral presentation at the AAPG InternationalConference and Exhibition, Rio de Janeiro,November 15–18, 2009.

3. Montaron and Tapponnier, reference 2.4. Hardie LA: “The Roles of Rifting and Hydrothermal

CaCl2 Brines in the Origin of Potash Evaporites: AnHypothesis,” American Journal of Science 290, no. 1(January 1990): 43–106.Hardie LA: “On the Significance of Evaporites,”Annual Review of Earth and Planetary Sciences 19(May 1991): 131–168.Warren JK: Evaporites: Sediments, Resources andHydrocarbons. Berlin: Springer-Verlag, 2006.

5. Montaron and Tapponnier, reference 2.6. Montaron and Tapponnier, reference 2.

52 Oilfield Review

of gas on a 5/8-in. choke, producing light oil with adensity of about 880 kg/m3 [30° API gravity] anda low sulfur content of about 0.5%.27 Developmentdrilling in the field confirmed the operator’s esti-mates of up to 1,000 million m3 [6.5 billion bbl] ofrecoverable oil, thus drawing worldwide atten-tion to Brazil’s presalt play.28 Many subsequentpresalt discoveries have been made in the Santosand Campos basins of Brazil.

In 2012, the Azul-1 well by Maersk Oil andthen the Cameia-1 well by Cobalt InternationalEnergy, Inc., extended the proven presalt playacross the South Atlantic to the Kwanza basin,offshore Angola.29 The Azul-1 well was in 953 m[3,130 ft] of water in Kwanza basin Block 23; thewell was drilled to 5,334 m [17,500 ft] and dem-onstrated potential flow capacity of greater than3,000 bbl/d [480 m3/d] of oil. The Cameia-1 well

was in 1,682 m [5,518 ft] of water in Kwanzabasin Block 21; the well was drilled to 4,886 m[16,030 ft] and flowed 5,010 bbl/d [800 m3/d] ofoil and 14.3 MMcf/d [405,000 m3/d] of gas.

In the process leading up to the Cameia-1discovery, exploration experts at CobaltInternational Energy recognized that during theAptian age, the present-day Kwanza and Campospresalt basins were in the same depositionalbasin, separated by only 80 to 160 km [50 to100 mi]; explorationists concluded the basinsmust have shared the same presalt history andhave similar characteristics.30 The presalt playthat led to the Tupi discovery in the BrazilianSantos basin was extended north along theBrazilian coastline to the Campos basin. Cobaltdrilled the Cameia-1 well to hunt for a Camposbasin presalt play analog across the AtlanticOcean in the Kwanza basin offshore Angola. TheCameia-1 oil discovery well drilled into a reser-voir that contained high-quality, highly perme-able and fractured carbonates in postrift andpresalt strata atop a basement high and wassealed by salt. The well encountered an oil col-umn that was about 370 m [1,200 ft] thick andcontained more than 270 m [900 ft] of net pay.31

To appraise the discovery, Cobalt drilled theCameia-2 well and confirmed the vertical and lat-eral extent, geometry and quality of the reservoir(left). The appraisal well validated the Cobaltmodel of additional reservoirs within the postriftand synrift strata beneath the original discoveryand indicated the reservoirs were separated byseals. Cobalt is conducting ongoing testing todetermine reservoir potential—the number ofreservoirs and seals, how the fluids vary betweenthe reservoirs, the reservoir properties and thedepths to the oil/water contacts.32

Matching Turbidite Sequences:From Ghana to French GuianaThe West Cape Three Points partnership discov-ered the Jubilee oil field offshore Ghana in June2007. The partnership comprises Kosmos EnergyLtd., Tullow Oil plc, Anadarko PetroleumCorporation, Sabre Oil & Gas, Inc., GhanaNational Petroleum Company and EO Group Ltd.The Mahogany-1 discovery well encountered 90 m[300 ft] of high-quality pay in an upper Cretaceousturbidite reservoir confined by a combinationstructural-stratigraphic trap.33 In August 2007, theHyedua-1 well, located 5.3 km [3.3 mi] southwestof the Mahogany-1 discovery, encountered 41 m[130 ft] of high-quality reservoir in equivalent tur-bidite sandstones. These wells opened up a deep-

> Kwanza basin presalt prospects and discoveries. The Cobalt Cameia-1 and Cameia-2 wellsdiscovered and appraised, respectively, oil reservoirs in the synrift (light brown) and postrift (yellow)sedimentary basins under the autochthonous salt (purple)—the presalt sediments—in Block 21(center right ), Kwanza basin offshore Angola. Cobalt plans to drill the Lontra, Idared, Mavinga andBicuar wells (dashed lines) to test other prospects in Blocks 20 and 21. The Cameia-1 well discovereda superpay reservoir (bright green) atop a basement high (bottom). Cobalt drilled the Cameia-2 well,a step-out well, to confirm the size of the discovery and to explore prospective reservoir zones belowthe superpay reservoir. The appraisal well confirmed the discovery and underlying reservoir intervals(light green), which are separated by sealing intervals (red). (Illustrations used with permission fromCobalt International Energy, Inc., reference 32.)

Lontra

Block 20

Idared Mavinga Cameia-1 Cameia-2 Bicuar

Block 21SouthNorth

SaltSalt

BasementSynrift Synrift Synrift

SynriftSynrift

Basement

Postsalt

Postrift

Postrift Postrift

PostriftPostrift

SaltSalt

Superpay reservoir

Middle reservoir

Lower reservoir

Cameia-1 Cameia-2

AFRICA

20

21

Angola

Postsalt Postsalt

Postrift

Oil confirmed by production

Oil confirmed by log or oil sample

Untested possible oil zone

Seal

Autumn 2012 53

27. “BG, Petrobras Announce Discovery of Oil Field inSantos Basin Offshore Brazil,” Drilling Contractor 62,no. 6 (November–December 2006): 8.

28. “Country Analysis Briefs: Brazil,” US Energy InformationAdministration (February 28, 2012), http://www.eia.gov/countries/cab.cfm?fips=BR (accessed August 29, 2012).

29. “Maersk Oil Strikes Oil with Its First Pre-Salt Well inAngola,” Maersk Oil (January 4, 2012), http://www.maerskoil.com/Media/NewsAndPressReleases/Pages/MaerskOilstrikesoilwithitsfirstpre-saltwellinAngola.aspx (accessed March 29, 2012).“Cobalt International Energy, Inc. Announces SuccessfulPre-Salt Flow Test Offshore Angola,” CobaltInternational Energy, Inc. (February 9, 2012),http://ir.cobaltintl.com/phoenix.zhtml?c=231838&p=irol-newsArticle&ID=1659328&highlight (accessed April4, 2012).

30. Cobalt International Energy, Inc.: “Update on West Africaand Gulf of Mexico Drilling Programs,” (February 8,2012), http://phx.corporate-ir.net/External.File?item=UGFyZW50SUQ9MTI1NzQyfENoaWxkSUQ9LTF8VHlwZT0z&t=1 (accessed August 2, 2012).Dribus JR: “Integrating New Seismic Technology andRegional Basin Geology Now a Must,” Journal ofPetroleum Technology 64, no. 10 (October 2012): 84–87.

water play targeting reservoirs in Late Cretaceousturbidites along the equatorial African transformmargin, which stretches from northern SierraLeone east to southern Gabon in the equatorialsegment of the South Atlantic Ocean.

Deepwater turbidite fields discovered off-shore Ghana are charged with hydrocarbonssourced from organic-rich sediments that rap-idly filled deep, active pull-apart basins duringthe Early Cretaceous epoch (above). Thesebasins formed on rifted continental crustbetween transform faults. During the Albian age,the continents split and seafloor spreadingbegan. Oblique motion between the two marginsis recorded by transform faults and fracture

> Opening of the equatorial Atlantic Ocean. Rifting between northern South America and southern West Africa started during the Early Cretaceous about125 Ma (top left). Small basins opened when continental crust stretched, thinned and faulted. These basins filled with sediment from the eroding continentaluplands and were deformed along the transform fault zones. During the Late Aptian to Early Albian, about 110 Ma (bottom left), oceanic spreading andaccretion began. Ocean floors grew as the plates were separating during the Late Albian, about 100 Ma (top right). By Late Santonian to Early Campanian,about 85 Ma (bottom right), the continental separation was complete. The seafloor spreading and passive margin phase began and the steep transformmargins subsided thermally and were cut, loaded and blanketed by river and delta sediments from the continents while South America and Africa continuedto separate. (Adapted from Brownfield ME and Charpentier RR: “Geology and Total Petroleum Systems of the Gulf of Guinea Province of West Africa,“Reston, Virginia, USA: US Geological Survey Bulletin 2207-C, 2006.)

Benuetrough

Benin andKeta basins

Voltabasin

Bové basin

Ivory Coastbasin

Senegal basin AFRICA

SOUTH AMERICA

Voltabasin

Senegal basin

Ivory Coastbasin

Benin andKeta basins

Benuetrough

Para-Maranhaobasin

15˚W 10˚W 5˚W 0˚ 5˚E

15˚W 10˚W 5˚W 0˚ 5˚E

10˚N

5˚N

15˚W 10˚W 5˚W 0˚ 5˚E

10˚N

5˚N

Voltabasin

Benin andKeta basins

Benuetrough

Ivory Coastbasin

Senegal basin

Para-Maranhaobasin

~

~

Bové basin

Bové basin

Para-Maranhaobasin

10˚N

5˚N

15˚W 10˚W 5˚W 0˚ 5˚E

10˚N

5˚N

0 500 km

0 300 mi

0 500 km

0 300 mi

0 500 km

0 300 mi

0 500 km

0 300 mi

Senegal basin

Voltabasin

Ivory Coastbasin

Benin andKeta basins

Benuetrough

Ocean

Bové basin

Para-Maranhaobasin

AFRICA

SOUTH AMERICAEarly Cretaceous, 125 Ma Late Albian, 100 Ma

Late Aptian to Early Albian, 110 Ma Late Santonian to Early Campanian, 85 Ma

West African shield

Brazilian shieldOnshore Mesozoic to Cenozoiccoastal basins

Thick continental crustand extension

Divergent basins, thinnedcontinental crust and thick clastics Direction of crustal extension

Transform fault zones

Present-day 2,000-m [6,560-ft] isobath

Zaedyus discovery,Guyane Maritime, French Guiana

Jubilee discovery,Tano basin, Ghana

AFRICA AFRICA

SOUTH AMERICASOUTH AMERICA

OceanOcean

OceanOcean

~

~

31. Cobalt International Energy, Inc.: “InvestorPresentation—March 2012,” (March 13, 2012),http://phx.corporate-ir.net/phoenix.zhtml?c=231838&p=irol-presentations (accessed June 8, 2012).

32. “Multiple Catalysts To Grow Shareholder Value,” CobaltInternational Energy, Inc. (September 19, 2012), http://phx.corporate-ir.net/External.File?item=UGFyZW50SUQ9NDgwMTA3fENoaWxkSUQ9NTEzNzk4fFR5cGU9MQ==&t=1 (accessed September 20, 2012).

33. A turbidite is a rock deposited from a turbidity flow,which is an underwater current of sediment-laden waterthat moves rapidly down a slope. The gravity, or density,current moves downslope because its density is higherthan that of the surrounding water.Dailly P, Henderson T, Hudgens E, Kanschat K andLowry P: “Exploration for Cretaceous Stratigraphic Trapsin the Gulf of Guinea, West Africa and the Discovery ofthe Jubilee Field: A Play Opening Discovery in the TanoBasin, Offshore Ghana,” in Mohriak WU, Danforth A,Post PJ, Brown DE, Tari GC, Nemcok M and Sinha ST(eds): Conjugate Divergent Margins. London: TheGeological Society, Special Publication 369, http://dx.doi.org/10.1144/SP369.12 (accessed August 7, 2012).

54 Oilfield Review

zones, and subsidence and sediment depositionoccurred during rifting and subsequent sag ofthe margins (above).

The opening and deepening of the equatorialSouth Atlantic and the global rise and fall of sealevel controlled sedimentation after continentalbreakup. Erosion of the continent led to deposi-tion of sediments in deltas on the continentalmargins. When sea level fell—a lowstand—therivers cut through their deltas and carried sedi-ments, often in sediment avalanches known asturbidity currents, onto the steep continentalslopes and toward the deep abyssal plain. Sandsthat were deposited as these turbidity currentsslowed may have formed reservoirs for deepwateroil fields such as those of the upper Cretaceousseries in the Jubilee field. Subsequent depositionof muds sealed these reservoirs as they were

buried beneath thousands of meters of youngersediment. During the Late Cretaceous epoch, themovement of the tectonic plates changed direc-tion, causing deformation of the rifted marginand the formation of structures that helped formtraps, and oil started migrating updip toward thecoast (next page, bottom right).34

The partnership drilled the Mahogany-1 wellto reservoir rock in a Turonian-stage stack of low-stand turbidite sands on the SW flank of theSouth Tano ridge.35 The reservoir was3,530 to 3,760 m [11,600 to 12,300 ft] below theseafloor. A drillstem test demonstrated that thewell was capable of flowing oil at 20,000 bbl/d[3,200 m3/d]. The oil was sourced from EarlyCretaceous rift-related organic-rich shales. TheJubilee well proved the Late Cretaceous turbi-

dite play concept and subsequent drillingrevealed that Jubilee is part of a collection offields offshore Ghana that includes Tweneboa,Enyenra and Ntomme.

Similar Late Cretaceous turbidite reservoirsoccur along the entire equatorial African coast,which have led to additional oil discoveries suchas the Akasa and Teak fields offshore Ghana, thePaon field offshore Côte d’Ivoire and the Venus,Mercury and Jupiter fields offshore Sierra Leone.

Tullow Oil sought to project the Jubilee playto the transform margin of South America andduplicate the company’s deepwater success.36

Exploration experts at Tullow Oil used the prin-ciples of plate tectonics, followed the major frac-ture zones across the equatorial Atlantic andidentified basins offshore South America thatdisplayed similar elements of the Jubilee play.

> Conjugate transform margins. These seismic lines cross the Suriname–French Guiana (above) and Côte d’Ivoire–Ghana (nextpage, top) transform margins; the red dots on the globes are the locations of these seismic sections. The red lines mark theapproximate position of the Demerara Fracture Zone (FZ) and the Romanche FZ, on the left and right, respectively. Transformmargins are characterized by shallow dipping, often narrow, continental margins, bordered by marginal ridges that backstopsteep continental slopes across abrupt continent-ocean boundaries leading to oceanic abyssal plains. Explorers are targetingreservoirs located in abyssal plain sediments in upper Cretaceous turbidites that lie on top of lower Cretaceous organic-richsource rocks. The green dots mark the approximate stratigraphic position of these upper Cretaceous reservoirs. TheseCretaceous source and reservoir rocks are sealed and buried under marine shales. On the Côte d’Ivoire–Ghana seismic line,the labels A through F represent stratigraphic units identified from seismic data. [Adapted from Greenroyd CJ, Peirce C, Rodger M,Watts AB and Hobbs RW: “Demerara Plateau—The Structure and Evolution of a Transform Passive Margin,” Geophysical JournalInternational 172, no. 2 (February 2008): 549–564.]

330 340 350 360 370 380 390 400

Suriname–French Guianaabyssal plain

Continentalslope

Marginalridge

Demerara Plateau

Offset, kmSW NE

Autumn 2012 55

They found evidence for an upper Cretaceousseries of lowstand turbidite channels and fansdeposited during seafloor spreading and buriedunder a thick sequence of marine shales. Theyinferred the presence of Cretaceous source rocksand stratigraphic traps, buried and sealed by themarine shales. This led the exploration teams tofocus on the continental slope off the Guyana

34. Antobreh AA, Faleide JI, Tsikalas F and Planke S:“Rift–Shear Architecture and Tectonic Development ofthe Ghana Margin Deduced from Multichannel SeismicReflection and Potential Field Data,” Marine andPetroleum Geology 26, no. 3 (March 2009): 345–368.

35. Dailly et al, reference 33.36. Patel T: “Did the Continental Drift Create an Oil

Bonanza?: Tullow Oil Bets Huge Fields Are ‘Mirrored’Across the Atlantic,” Bloomberg Businessweek(February 24, 2011), http://www.businessweek.com/magazine/content/11_10/b4218020773519.htm(accessed August 20, 2012).

2030405060708090

Marginalridge

Deep Ivorian basin

Offset, km

Continentalslope

Gulf of Guineaabyssal plain

E

F

D

C

BA

> Reservoirs in Late Cretaceous turbidites. Explorationists looked for canyons feeding reservoir rocks inchannel-levee and turbidite fan deposits on the basin floor that originated from the Guyana ContinentalShelf and slope. These reservoir rocks are sourced and charged by Early Cretaceous organic-richshales that were deposited during continental rifting. Since their deposition, these reservoir rocks havebeen buried and sealed by marine shales (not shown). Expected well log responses are plotted for thefive types of deposits (boxed red areas between black curves); the left curve is spontaneous potentialor gamma ray, and the right curve is resistivity. (Illustration used with permission from Tullow Oil plc.)

Canyon fed by activenearshore littoral driftor relict shelf sands

Sandy coastalplain

Barrier bar

Inner fanchannels

Midfanchannelized

lobes

Slumpscar

Inner fan

Basin plain

Basin plain

Coastalplain

Continentalshelf

Slopeapron

Slumps

Slump

10 to 50 km5.4 to 27 mi

Longshoredrift

Slumpscar Outer fan

Midfan channelized andunchannelized sands

Shelf anddelta

500 to 2,000 m[1,640 to 6,562 ft]

S N

56 Oilfield Review

Shelf and east of the Demerara Plateau offshoreFrench Guiana (below).37

Tullow Oil and partners acquired 2,500 km2

[970 mi2] of high-quality 3D marine seismicdata over the steep continental slope offshoreFrench Guiana.38 Explorers at Tullow Oil usedthese data to look for submarine canyons andturbidite deposits on the basin floor that origi-nated from the Guyana Continental Shelf andslope. These seismic data showed features simi-lar to those observed in 3D seismic data over theJubilee field offshore Ghana. The explorationteam identified and mapped a number of pros-pects (next page). After follow-up regionalinvestigations, the Tullow Oil team decided totest the play by drilling a well at the GM-ES-1location within the Zaedyus prospect, in theGuyane Maritime license, which is about 150 km[93 mi] offshore.39

Tullow Oil started operations in March 2011,drilling near the toe of the continental slope in2,048 m [6,719 ft] of water. By September 2011,the company announced the discovery of 72 m[240 ft] of net oil pay within two turbidite fans.40

Wireline logs and samples of reservoir fluidsshowed good quality reservoir sands at a reser-voir depth of 5,711 m [18,740 ft]. The Zaedyusexploration well proved that the Jubilee play—developed for the transform margin offshoreGhana and applied successfully elsewhere alongthe equatorial African margin—was also appli-cable to the transform margin offshore FrenchGuiana and probably elsewhere along the trans-form margin of northern South America.

Learning from SuccessThe recent history of oil discovery along theSouth Atlantic margins has been one of learningfrom success. Pioneering explorationists studiedthe large discoveries of the Lula reservoir in theSantos basin, offshore Brazil, and the Jubileereservoir, offshore Ghana, and stepped along thesame margin to look across the ocean where con-jugate margins hosted similar large discoveries.

Explorationists used the principles of platetectonics to leverage their accomplishments.When a continent splits and a new spreadingcenter opens up, plate tectonic concepts pro-vide the basis for hypothesizing which series oftectonic and stratigraphic events will occur.Armed with the principles of plate tectonics andastute observations from exploration plays thathave led to successful discoveries, exploration-ists have extrapolated plays into new leads,

Oceanic transform fracture zone

FrenchGuiana

Guyana

SierraLeone

GhanaCôte d’IvoireLiberia

Equatorial Atlantic transform marginSuriname

Mid-Atlantic Ridge

SOUTH AMERICA

WEST AFRICA

Oceanic transform fracture zone

0 600 km

0 300 mi

West CapeThree Points block

DeepwaterTano block

Jubilee discovery

Oil discoveryGas condensate and oil discoveryProspectDry holeOil shows

0 25 km

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GuyaneMaritimelicense

Zaedyus discovery

DiscoveryProspectLead

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0 50 mi

> Extending West African success across to South America. Tullow Oil plc used plate tectonicconcepts to develop an exploration program to extend the Jubilee play (black star) proved alongthe West Africa transform margin to the northern South America transform margin. The transformmargins (gray shading) on the west and east sides of the Equatorial Atlantic have similar geology.Explorationists had recognized Late Cretaceous stratigraphic traps within the Guyana-Suriname basinthat were analogous to those proved by the Jubilee and similar discoveries in West Africa. Tullowexplorationists made the Zaedyus discovery in the Guyane Maritime license, offshore French Guiana(red star). (Illustration adapted with permission from Tullow Oil plc.)

Autumn 2012 57

prospects and drilling targets both regionallyand globally.

Understanding plate tectonics also allowsexplorationists to take what they learn from oneplay and ask, “What if?” If hydrocarbons are foundin an immature rift margin setting, could one findthe same in a mature rift margin or a transformmargin setting? In recent years, exploration com-panies have answered these questions affirma-tively through discovery wells. Recent discoveriesin the Albert rift basin of Uganda, the East Africa

rift basin of Kenya, the Levant basin offshoreIsrael and Cyprus and the Mozambique basin off-shore Tanzania have been similarly impressive.Plate tectonic concepts and models, and theirability to engender reasoned hypotheses for newplays, are powerful exploration tools for hithertoundeveloped basins. They are also cause forreexamining basins that have been exploredbut deemed either hydrocarbon poor or too riskyto develop. —RCNH

37. Plunkett J: “French Guiana—A New Oil Province,”presented at the Kayenn Mining Symposium, Cayenne,French Guiana, December 1–3, 2011.

38. The partnership was a joint venture between Tullow Oilplc—the operator—Royal Dutch Shell, Total andNorthpet, a company owned 50% by Northern Petroleumplc and 50% by Wessex Exploration plc. Royal DutchShell formally took over as operator of the GuyaneMaritime license on February 1, 2012.

39. Plunkett, reference 37.40. “Zaedyus Exploration Well Makes Oil Discovery

Offshore French Guiana,” Tullow Oil plc (September 9,2011), http://www.tullowoil.com/index.asp?pageid=137&newsid=710 (accessed August 10, 2012).

> Jubilee analogs offshore French Guiana. Tullow Oil plc acquired 2,500 km2 [970 mi2] of 3D seismic data in 2009 (red box in map inset). The depth-basedseismic interpretation image (top), viewed from above and the northeast, shows an Early Cretaceous horizon (color-coded in red to blue from shallow todeep) overlain by a Late Cretaceous horizon (brown to yellow) intersecting at the steep continental slope formed by the transform margin. The datarevealed features similar to those observed in the Tano–West Cape Three Points area, offshore Ghana. These features include a turbidite feeder canyonand structural high that focus sediments into channels and fan systems that are prospects for reservoirs. The close-up view of the area (bottom) showschannels and turbidite fans imaged by the 3D seismic data. (Images used with permission from Tullow Oil plc.)

Turbidite feeder canyon

Fan systems

Major turbidite fan

Structural high

Channels

Channel

Atlantic Ocean

GuyaneMaritimelicense

Zaedyus discovery

DiscoveryProspectLead

0 100 km

0 50 mi

Early Cretaceoushorizon

Late Cretaceoushorizon

Seismic horizon relationship

View angle